Sculpey, SuperSculpey and Sculpey III are the most brittle polymer clay brands/lines after baking any place they're thin or projecting (whole round balls, etc., will be inherently strong shapes).

The strongest brands/lines of polymer clay these days are Kato Polyclay (Hobby Lobby and online), Premo**, FimoClassic, Cernit, SuperSculpey-Firm (only in gray), and FimoSoft to a lesser degree.

(I have no idea what happened to your Fimo since no polymer clay should ever get softer over time... they only get stiffer --because they've cooled after being handled, ot they age naturally a bit which is called "advancing," or they partially cure from too high a heat or too much UV light. Polymer clays can begin to "dissolve" though if they're in contact with anything containing a petroleum-based solvent --which can begin to happen even up to 6 months later. A few plastic wraps will do that, and many stiff plastics will too. I've been involved with polymer clay in a big way for over ten years and haven't heard of it softening with age, so I'd be really curious about what happened!)

You can read a lot of details about all the brands and lines of polymer clay at my online polymer clay "encyclopedia" if you're interested:
http://glassattic.com/polymer/Characteristics.htm
(in addition to any particular brand or line, you might also want to read the category called "Strength --Ridigity & Flexibility")
P.S. Oops, I forgot there's still a problem with that particular page at my website when viewed with Firefox (3 at least)! The info is all there, but hard to read because of a margin problem we're still trying to figure out.

P.P.S. If you're interested in making jewelry in particular, here are some of the other pages at the site that deal with that topic (and there's no problem with their formatting, even with Firefox):
http://glassattic.com/polymer/beads.htm
and http://glassattic.com/polymer/beads-holes.htm
http://glassattic.com/polymer/jewelry.htm
http://glassattic.com/polymer/pendants_cording.htm
http://glassattic.com/polymer/buttons.htm
http://www.glassattic.com/polymer/wire.htm

Oh, and to make sure you're baking correctly (because incomplete curing can make any polymer clay breakable), check out this page on Baking the clay:
http://glassattic.com/polymer/baking.htm

**Premo had problems with their latest reformulation (making it way too sticky), but it's been corrected now... some may still be on the shelves though.

HTH,

Diane B.